1. Field of the Invention
The present invention is broadly concerned with ultrasound phantoms which are used to test the accuracy of and to calibrate ultrasonic diagnostic equipment presently in use by many hospitals and doctors. More particularly, it is concerned with an ultrasound phantom which has ultrasound characteristics (i.e., the transmission pattern of ultrasonic waves directed therethrough) in a manner to closely mimic the transmission pattern of similar ultrasonic waves directed through one or more portions of the human body; i.e., important ultrasound characteristics such as wave velocities, and attenuation and scattering coefficients of the phantom of the invention are very similar to those of human tissue.
2. Description of the Prior Art
Although diagnostic ultrasound equipment has been in use for a number of years, a persistent problem has plagued both the designers and users of such equipment. Specifically, a truly stable and uniform ultrasound phantom has not been available for calibrating and checking the equipment on a regular basis. Similarly, a tissue-mimicking ultrasound phantom having necessary stability and uniformity qualities has not heretofore been produced. Ideally, such a tissue-mimicking phantom should have the same ranges of uniform velocities of sound, attenuation coefficients, and scattering coefficients as human tissue, although for calibration purposes exact tissue-mimicking characteristics are not absolutely necessary.
A number of attempts have been made in the past to provide an effective ultrasound phantom. One such attempt is described in an article entitled "Tissue Mimicking Materials For Ultrasound Phantoms", by Ernest L. Madsen et al., Med. Phys., 5 (5), September/October 1978. In the phantoms described in this article, water-based pharmaceutical gels containing uniform distributions of graphite powder and known concentrations of alcohol are employed. One drawback in this type of ultrasound phantom stems from the fact that the graphite tends to settle out at temperatures over 90.degree. F., thus irreversably altering the ultrasound properties of the phantom. Moreover, many gels employed can be unstable under certain conditions, primarily due to bacterial attacks on the gel and ambient temperature variations, thus leading to degradation of the gel and consequent failure of the phantom. Finally, it is difficult to achieve and maintain a uniform dispersion of the graphite, and to incorporate zones therein for the mimicking of cysts or the like.
Another known ultrasound phantom produced by researchers at the University of Colorado employs a base of silicone polymer combined with mineral oil, polystyrene or glass beads embedded therein. Various other substances are being investigated as phantom materials, and these include soft plastics such as plastisols, or urethane polymers. However, the phantoms produced to date are generally deficient in one or more important respects. For example, it is very difficult to remove air bubbles from the material and achieve reproducibly uniform concentrations of scattering particles.